The present invention relates to a logging-while-drilling apparatus and method which utilize the drilling fluid as a communication link for telemetering a downhole signal to the surface and more particularly relates to a logging-while-drilling apparatus and method which include a means for encoding downhole information into an acoustical pressure wave signal by phase-shifting said signal.
The desirability of a system which is able to measure downhole drilling parameters and/or formation characteristics and transmit them to the surface while actual drilling of an earth well is being carried out has long been recognized. Several such systems have been proposed and are commonly referred to as "logging-while-drilling" systems. In logging-while-drilling systems, one of the major problems exists in finding the communication link necessary for telemetering the information from a downhole location and having it arrive at the surface in a meaningful condition.
In this regard, it has been proposed to telemeter the desired information by means of an acoustical pressure wave signal generated in and transmitted through the circulating mud system normally associated with well drilling operations. The pressure wave signal which is representative of a downhole condition is generated in the mud downhole near the bit by a signal-generating means and the wave travels up the hole through the mud to a signal processor at the surface. One logging-while-drilling system utilizing this basic type of telemetry is disclosed and fully described in U.S. Pat. No. 3,309,656 to John K. Godbey, issued Mar. 14, 1967.
In telemetering downhole information by means of an acoustical pressure wave signal carried through the mud as mentioned above, one important consideration is how the information is actually encoded into the signal. A method and apparatus is disclosed and fully described in applicant's copending United States application Ser. No. 213,061, filed December 28, 1971, now U.S. Pat. No. 3,789,355, issued Jan. 29, 1974, for encoding information to such a signal wherein the operation of the downhole signal-generating means is controlled in response to downhole information so that the signal being generated is phase-shifted to denote the information being encoded therein. The apparatus of the present invention also encodes downhole information into an acoustical pressure wave by phase-shifting said signal but utilizes different method and means within the downhole apparatus for carrying out this function.